Display device having plasmonic color filters and photovoltaic capabilities

ABSTRACT

A plasmonic optical spectrum filtering device is provided that filters electromagnetic waves by optical resonance, for example, by selective conversion between the free-space waves and spatially confined modes in plasmonic nano-resonators. Frequency-selective transmission and reflection spectra are engineered and can be used as spectrum filters for display and imaging applications. A thin film stack color filter is further disclosed, which can be designed to either function as a transmission color filter with efficiency twice that of conventional colorant based color filter; or as a reflective color filter for display devices (e.g., used in an energy harvesting reflective display). In other variations, a novel reflective colored display is viewable under direct sunlight, and can simultaneously harvest both incident light and generate electrical power. Methods of making such plasmonic optical spectrum filtering devices are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/095,365 filed on Apr. 27, 2011, and claims the benefit of U.S.Provisional Application No. 61/328,335 filed on Apr. 27, 2010. Theentire disclosures of each of the above applications are incorporatedherein by reference.

FIELD

The present disclosure relates to spectrum filtering devices for displaydevices such as liquid crystal displays, projection displays like liquidcrystal on Si (LCoS), eye-wear displays, as well as plasmonic colorfilters, which can also have photovoltaic capabilities.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Display devices occupy an increasingly large fraction of surface area inmany computing devices. Energy efficiency is becoming ever moreimportant for a green and sustainable future. As global energy demandcontinues to grow to meet the needs and aspirations of people across theworld, ways to improve energy efficiency and harvest energy areessential. However, little attention has been paid to the significantlight energy wasted in displays used in everyday lives. For example, inthe prevailing liquid crystal displays (LCD), only 3-8% of the backlightreaches a viewer's eyes, where most of light energy is absorbed by thecolorant-based filters and polarizers.

Color and spectral imaging systems in display devices typically usefilters and glass prisms to disperse light of different wavelengths.Transmissive optical filters are widely utilized in various displayapplications, including flat panel screens, like liquid crystal display(LCD) panels. Optical wavelength filters are devices that reflect ortransmit light of a desired wavelength or within a certain wavelengthrange. For example, a transmissive filter selectively transmits lightwithin a preselected wavelength transmission bandwidth, while absorbingor reflecting light of wavelengths outside the transmission bandwidth.Such optical filtering for wavelength provides a way to control theenergy and spectral composition of light and is widely used in displayapplications.

To produce color images, existing display devices produce three primarycolors, typically, red, green, and blue, collectively referred to as“RGB.” Conventional optical filters use pigment dispersions to filterand produce RGB colors for display pixels. Light of complementary colorsare absorbed and completely wasted. Such optical filters are typicallymanufactured by four separate processes, which not only complicatesmanufacturing, but also wastes chemical materials in the process. Thus,such optical filters have relatively low energy efficiency, while addingsignificantly to the overall cost and size of the display device.Currently used polarizers in LCD displays achieve the polarizationfunction by absorbing light of the orthogonal polarization and theabsorbed light is also wasted.

With the miniaturization of integrated devices, there is a need for anew paradigm in color filter technology that can produce optical filtersin the visible range with higher transmission efficiency and reducedmanufacturing complexity to provide devices with high energy efficiency,low power consumption, and slim dimension. Furthermore, it would bedesirable to have display devices capable of recycling or harvestingabsorbed energy to generate useful electrical power, especially fordevices such as electronic books that consume relatively small amountsof power, or mobile devices (such as cell phones) that are in standbymode 95% of the time.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

According to various aspects of the present teachings, a plasmonicoptical spectrum filtering device is provided that comprises a resonatorstructure. The resonator structure comprises an electrically conductivemetal grating structure and a dielectric material. The electricallyconductive metal grating structure comprises at least two openingscapable of transmitting a portion of an electromagnetic spectrum togenerate a filtered output having a predetermined range of wavelengths.The filtering occurs at least in part via an optical resonance process.

In other aspects, the principles of the present disclosure provide adisplay device comprising a display pixel of a display screen. Thedisplay pixel comprises a plasmonic resonator structure for colorfiltering via optical resonance. The plasmonic resonator structurecomprises an electrically conductive metal grating structure and anactive material, such as a photoactive material or a dielectricmaterial. The electrically conductive metal grating structure comprisesat least two openings capable of transmitting a portion of anelectromagnetic spectrum generated by the display device. Theelectromagnetic waves can transmit through the two or more openings togenerate a filtered and polarized output having a predetermined range ofwavelengths via optical resonance. In certain variations, such a displaydevice is a liquid crystal display (LCD) device. The resonator structurecan serve as a transparent conductive electrode and a polarizer in apixel for such an LCD display.

In certain variations, a novel reflective colored display is providedthat is viewable under direct sunlight and can simultaneously harvestincident light to generate photocurrent and thus electrical power.Moreover, a plasmonic optical spectrum filtering device comprising aresonator structure, such as a thin film stack based color filterplatform, is disclosed, which can be designed to either function as atransmission color filter with efficiency twice that of conventionalcolorant based color filter; or as a reflective color filter as will beused in the proposed energy harvesting reflective display.

In yet other variations, a method of spectrum filtering is provided thatcomprises filtering an electromagnetic spectrum by optical resonance ofa plasmonic resonator structure. The resonator structure comprises anelectrically conductive metal grating structure and an active materialselected from a dielectric material or a photoactive material. Theresonator structure can thus generate a filtered output having apredetermined range of wavelengths. In accordance with certainprinciples of the present teachings, a periodicity of the electricallyconductive grating structure relates to the predetermined range ofwavelength(s) that is transmitted. For example, in certain embodiments,the predetermined range of wavelengths of the filtered output is in thevisible light range and has a color selected from the group consistingof: cyan, yellow, magenta, red, green, blue, and combinations thereof.

Furthermore, the present disclosure also provides in various aspects,methods of making plasmonic optical spectrum filtering devices. Suchmethods include forming a resonator structure comprising an electricallyconductive metal nanograting subwavelength structure and an activematerial selected from a dielectric material or a photoactive materialvia a process selected from UV photolithography, nanoimprintlithography, focused ion beam processing, stamping or metal transferprinting. In this manner, the electrically conductive metal nanogratingsubwavelength structure is formed that comprises at least two openingscapable of transmitting a portion of an electromagnetic spectrum therethrough to generate a filtered output having a predetermined range ofwavelengths via optical resonance.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 shows a sectional schematic of a conventional liquid crystaldisplay pixel.

FIG. 2 is a schematic of a thin metal grating on a substrate used tocreate a plasmonic resonator filtering device in accordance with certainprinciples of the present disclosure.

FIG. 3 is a sectional view of an exemplary inventive plasmonic resonatorfiltering device having a metal-insulator-metal (MIM) stack or arraywith sub-wavelength gratings formed in accordance with the presentteachings.

FIG. 4 is a sectional view of an exemplary inventive reflectiveplasmonic resonator filtering device with dual-function functioning asboth a color filter and photovoltaic solar cell having a resonatorassembly including a photoactive material and at least one grating inaccordance with certain aspects of the present teachings.

FIGS. 5A-5B show simulated reflective color filtering. FIG. 5A showsreflection spectra for red, green, blue (RGB) based on preliminarydesign and simulation of a reflective filtering device like that of FIG.4, while FIG. 5B shows absorption for such a device.

FIGS. 6A-6I are energy-generating photonic color filters. FIGS. 6A-6C isa schematic of dual-function devices. In all three cases, the thicknessof Au nanogratings and PEDOT:PSS layer are 40 nm and 30 nm respectively.Au nanogratings have 0.7 duty cycle (Au line width is 0.7 of theperiod). The photovoltaic property with an external current collector isschematically described in FIG. 6B as a representative. FIG. 6A is acyan colored device having 420 nm period Au nanogratings and 90 nmthickness P3HT:PCBM photoactive layer. FIG. 6B is a magenta coloreddevice having 280 nm period Au nanogratings and 65 nm thicknessphotoactive layer. FIG. 6C is a yellow colored device having 220 nmperiod Au nanogratings and 50 nm thickness photoactive layer. FIGS.6D-6F are photographs of dual-function devices having 1 mm diametercircular shape. The inset images are the large area version having about1 cm size; FIG. 6D is cyan, FIG. 6E is magenta, and FIG. 6F is yellow.FIGS. 6G-6I are scanning electron microscope (SEM) images of Aunanogratings. The left and right inset images are the high magnificationtop and tilted views, respectively. The “P” in the left inset imagerepresents the period of Au nanogratings. FIG. 6G is a 420 nm period,FIG. 6H is a 280 nm period, FIG. 6I is a 220 nm period.

FIGS. 7A-7D are calculated maps of the reflection for the proposedstructures. The thickness of Au nanogratings and PEDOT:PSS layer arefixed at 40 nm and 30 nm, respectively. Au nanogratings have 0.7 dutycycle. FIGS. 7A-7B is a reflection for transverse electric (TE)polarized light waves as a function of the thickness of P3HT:PCBM blendphotoactive layer (Au nanograting period is fixed at 280 nm) (FIG. 7A)and Au nanograting period (the thickness of photoactive layer is fixedat 50 nm) (FIG. 7B). FIG. 7C is a reflection for transverse magnetic(TM) polarized light waves as a function of Au nanograting period (thethickness of photoactive layer is fixed at 50 nm). FIG. 7D is a magneticfield intensity distribution for TM (top) and TE (bottom) waves at thesame resonant absorption wavelength of 490 nm. The thickness ofphotoactive layer and the period of Au nanogratings are 50 nm and 220nm, respectively.

FIGS. 8A-8F are color filtering behaviors in dual-function devices.FIGS. 8A-8C are the reflection spectra calculated by Rigorous CoupledWave Analysis (RCWA) simulation. The solid line, dashed line, and dottedline represent unpolarized condition, TM mode, and TE mode,respectively; FIG. 8A is cyan, FIG. 8B is magenta, and FIG. 8C isyellow. FIGS. 8D-8F are measured reflection spectra. Open circle,half-down open circle, and half-up open circle represent unpolarizedcondition, TM mode, and TE mode, respectively; FIG. 8D is cyan, FIG. 8Eis magenta, and FIG. 8F is yellow.

FIG. 9 shows photovoltaic behaviors in dual-function devices. Circle,square, and triangle symbols represent the devices showing cyan, magentaand yellow colors, respectively. J-V plots of dual function devices. Alldata were measured at AM 1.5 G with an intensity of 100 mW cm⁻². Averagesolar cell characteristics such as short circuit current density (Jsc),open circuit voltage (Voc), fill factor (FF) and power conversionefficiency (PCE) are summarized as follows: cyan (Jsc=5.28 mA cm⁻²,Voc.=0.51 V, FF=57.5%, PCE=1.55%); magenta (Jsc,=5.04 mA cm⁻², Voc=0.49V, FF=33.1%, PCE=0.82%); yellow (Jsc,=3.98 mA cm⁻², Voc,=0.48 V,FF=31.2%, PCE=0.60%). The inset image is a schematic of dual-functiondevice having a photovoltaic function.

FIGS. 10A-10D are plasmonic nanoresonators formed by a resonatorstructure having metal-insulator-metal (MIM) stack arrays. FIG. 10A is aschematic diagram of an exemplary embodiment of such an inventiveplasmonic nanoresonator. The white arrow represents a source of incidentwhite light and red, yellow, green and blue arrows represent filteredoutput of transmitted colored light from the different stack arrays. TheMIM resonator structure includes aluminum, zinc selenide and magnesiumfluoride materials, respectively. An inset is the scanning electronmicroscopy image of the fabricated device. Scale bar, 1 μm. FIG. 10Bshows plasmon dispersions in the MIM stack array. Red, green and bluedots correspond to the case of filtering primary RGB colors. Red andblue lines/curves correspond to antisymmetric and symmetric modesrespectively. A shaded region indicates the visible range. FIG. 10C issimulated transmission spectra for the RGB filters. The solid and dashcurves correspond to TM and TE illuminations respectively. The stackperiod for RGB filters is 360, 270 and 230 nm. FIG. 10D is across-section of the time-average magnetic field intensity and electricdisplacement distribution (red arrow) inside the MIM stack at awavelength of 650 nm with 360 nm stack period. On the right side of FIG.10D constitutive materials are indicated, in the same configuration asin FIG. 10A.

FIG. 11 is a schematic diagram of another exemplary embodiment of aninventive plasmonic nanoresonator filter device comprising a stack arrayof metal-insulator-metal (MIM) thin films, which generates filteredlight.

FIGS. 12A-12D are plasmonic color filters. FIG. 12A is opticalmicroscopy images of seven plasmonic color filters illuminated by whitelight. Scale bar, 10 μm. FIG. 12B is experimentally measuredtransmission spectra of three fabricated color filters corresponding tothe RGB colors. The circle and triangle correspond to TM and TEilluminations respectively. FIG. 12C is scanning electron microscopyimage of the pattern “M” formed by two stack periods. The periods of thenavy blue background and the yellow character are 220 and 310 nm,respectively. Scale bar, 3 μm. FIG. 12D is an optical microscopy imageof the pattern illuminated with white light.

FIG. 13 is a simulated transmission for green and red filters preparedaccording to the present teachings having 2, 4, 6, and an infinitenumber of openings or slits. The circle, triangle, diamond and starcorrespond to the structure with 2, 4, 6 and infinite slitsrespectively. An inset shows the optical microscopy images for the caseof 2, 4 and 6 slits (slit number increases from bottom to top).

FIGS. 14A-14D are plasmonic spectroscopes for spectral imaging preparedin accordance with certain aspects of the present teachings. FIG. 14A isa scanning electron microscopy (SEM) image of the fabricated 1Dplasmonic spectroscope with gradually changing periods from 400 to 200nm (from left to right). Scale bar, 21 μm. FIG. 14B is an opticalmicroscopy image of the plasmonic spectroscope illuminated with whitelight, showing a spectrum of filtered colors ranging from red to blue.FIG. 14C is an SEM image of a two-dimensional plasmonic resonator havinga spoke structure prepared in accordance with the certain aspects of thepresent teachings. Scale bar, 31 μm. FIG. 14D includes opticalmicroscopy images of the spoke structure of FIG. 14C illuminated withunpolarized light (center) and polarized light (four boxes), showing aspectrum of filtered colors (including blue “B,” cyan “C,” green “G,”yellow “Y,” and red “R”) from different angles.

FIGS. 15A-15C are TM transmission simulation results for certainplasmonic resonator filter embodiments according to the presentteachings, which includes a polarizing structure including a thickaluminum grating and a high refractive index dielectric material (FIG.15A), and a thin aluminum grating, lower refractive index dielectricmaterial as a high transmission structure (FIG. 15B). FIG. 15C is aschematic of an exemplary corresponding plasmonic resonator filterstructure.

FIGS. 16A-16B include a schematic of an alternative embodiment accordingto certain aspects of the present teachings, including a resonatorstructure that contains a low refractive index spacer layer of silicondioxide between a metal grating comprising gold and a high refractiveindex dielectric material guiding layer (Si₃N₄). FIG. 16B isexperimental TM transmission result obtained for such a structure withtwo different metal grating periods.

FIGS. 17A-17B. FIG. 17A is a schematic of an embodiment of a reflectivecolor filter structure according to certain aspects of the presentdisclosure. FIG. 17B shows reflection spectra for the structure of FIG.17A having varying periods under TM polarized illumination.

FIGS. 18A-18D is a schematic of a method of fabricating an exemplaryplasmonic resonator structure according to certain aspects of thepresent teachings via a pattern transfer process. FIG. 18A shows asurfactant coated silicon dioxide (SiO₂) mold. FIG. 18B shows anevaporation process of thin film metal layers (gold (Au), followed byaluminum (Al), then titanium dioxide (TiO₂)) followed by sputtering ofthick and continuous aluminum (Al). FIG. 18C shows press molding into apolycarbonate (PC) substrate with application of temperature/pressure.FIG. 18D is shows a detached PC sample separated from the SiO₂ mold.

FIGS. 19A-19B is an SEM image of yellow reflective filter structure with220 nm period.

FIGS. 20A-20D. FIG. 20A is a TE simulation and FIG. 20B is a TMsimulation (red, dashed) and data (black, solid) for yellow reflectivefilter structure prepared in accordance with certain aspects of thepresent disclosure having a 220 nm period along with microscope imagesof respective polarizations. FIG. 20C is a picture of a front of thefinal transferred structure on a PC substrate and FIG. 20D showsphotographs of the same. The back looks like a flat Al film while thefront shows a distinct yellow color. The drop-shaped object is a regionon the sample without the top Al grating, which provides a polarizationindependent reference for the optical images.

FIGS. 21A-21C are graphs showing angular dependence of TE (FIG. 21A), TM(FIG. 21B), and unpolarized reflectance spectra of yellow filter (FIG.21C). The angle between the source and detector varies from 20° to 50°.

FIG. 22 is a pixel array for a liquid crystal display panel;

FIG. 23 is a cross-sectional detailed view of an exemplary pixel fromFIG. 22 where a liquid crystal control layer is disposed over on top ofan inventive optical plasmonic resonator reflective color filter andpolarizer device that produces a colored pixel with on/off control inaccordance with certain aspects of the present teachings.

FIG. 24 is a sectional view of an exemplary inventive optical plasmonicresonator reflective color filter prepared in accordance with certainaspects of the present teachings that can be used in a liquid crystaldisplay pixel of FIGS. 22 and 23.

FIGS. 25A-25D show a high efficiency narrow band optical plasmonicresonator color filter having a buffer layer, a modulation layer, and asubwavelength grating having varying periods between grating rows(either 300 nm, 350 nm, or 450 nm) prepared in accordance with certainprinciples of the present disclosure (sectional view of design in FIG.25A). FIG. 25B shows sectional views of filtered output from such adevice, demonstrating the ability to filter blue, green, and red basedon different periods of the grating structure. FIG. 25C shows asimulation of wavelength versus transmission for the three colors andFIG. 25D shows experimental data of wavelength versus transmission forcomparison, where full width at half maximum (FWHM) is about 25-30 nm.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. Example embodiments will now be described more fully withreference to the accompanying drawings.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to” or “directly coupled to” another element orlayer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

According to the present disclosure, a novel spectrum filtering deviceis provided. “Spectrum filtering” broadly encompasses filtering of anywavelength range of electromagnetic radiation, while color filteringrefers to filtering within the visible light portion of theelectromagnetic spectrum. Spectrum filtering is used interchangeablyherein with the term “color filtering.” In various aspects, an opticalspectrum filtering device is provided that comprises a thin film stackarranged to form a resonator structure. Such a resonator structureaccording to certain aspects of the present teachings can be designed toeither function as a transmission color filter with efficiency twicethat of conventional colorant-based color filters; or as a reflectivecolor filter, which can be used in conjunction with an energy harvestingreflective display. For example, one embodiment is a plasmonic opticalspectrum filtering device for use as a color filter that comprises aresonator structure. Thus, the resonator structure comprises anelectrically conductive metal grating structure and an active material,such as a dielectric material, a photoactive material, or combinationsthereof. In certain embodiments, the resonator structure may include ametal-dielectric-metal stack. The electrically conductive metal gratingstructure comprises at least two openings (e.g., spaces between rows ofmaterial, for example) capable of transmitting a portion of anelectromagnetic spectrum to generate a filtered output having apredetermined range of wavelengths via optical resonance.

According to the certain aspects of the present teachings, a devicecomprising a metal-dielectric-metal resonator structure (also referredto herein as a metal-insulator-metal or “MIM”) is provided for aspectrum filtering function (e.g., color filtering) for bothtransmission and reflection type filters. In various aspects, the devicehas a sub-wavelength periodic structure that can efficiently couple theincident light into specific plasmon modes of the MIM stack and thenscatter them to the far field. The term “sub-wavelength” is generallyused to describe one or more dimensions smaller than a length of thewave that the device interacts with (for example, a sub-wavelengthgrating means a grating structure having openings for transmission withone or more dimensions that are less than a wavelength(s) of lightinteracting with it). By tuning a periodicity of the gratingstructure/resonator (e.g., a stack period or a period defined betweenopenings) and a dielectric layer thickness, transmission or reflectionpeaks can be tuned to cover the visible wavelength range.

In certain aspects, the present teachings provide an optical spectrumfiltering device capable of transmitting a portion of an electromagneticspectrum to generate a filtered output having a predetermined range ofwavelengths that exits the filter assembly. Both transmission andreflection spectrum or color filtering can be achieved. Thus, in certainvariations, the optical spectrum filtering device may be atransmission-type filter, while in other variations; the opticalspectrum filtering device may be a reflection-type filter. In yet othervariations, the optical spectrum filtering device concurrently exhibitsboth a transmission and reflection type filter.

By way of background, in recent years, surface plasmons (SPs) andrelated plasmonic nano-structures have generated considerable interestwith the development of nanofabrication and characterization techniques.SPs are essentially charge density waves generated by the coupling oflight to the collective oscillation of electrons on the metal surface.By exploiting plasmonic nanostructures, such as nanohole or nanoslitarrays, efficient conversion between photons and plasmons can becontrolled at sub-wavelength scale, which provides new solutions totraditional optical processes, such as color filtering and spectralimaging.

Recently, such effects have been reported using a metallic nanoholearray to filter color by tuning the resonant transmission peak (filteredoutput) at the visible range. However, the transmission passbands ofsuch filters are relatively broad and do not satisfy the requirementsfor multiband spectral imaging. Other attempts, such as nanoslitscombined with period grooves or inserted into a metal-insulator-metal(MIM) waveguide also show color filtering effect. However, in thesestructures, two neighboring output openings or slits must be separatedby additional structures or by specific coupling distances (both aboutseveral micrometers, causing attenuation due to metal absorption loss);therefore, the device dimension and efficiency are restricted. Moreover,because of the thick metal film used in these structures, the absorptionloss from light entering and leaving the MIM waveguide further decreasesthe devices' efficiencies to generally less than 10%. Such an efficiencyvalue is generally considered to be inadequate and does not satisfy therequirement for practical display applications.

MIM waveguide geometries offer the ability to support SP modes atvisible wavelengths and are useful in a variety of differentapplications, such as guiding waves at sub-wavelength scale,concentrating light to enhance the absorption for photovoltaicapplications, providing a near-field plate for superfocusing at opticalfrequency or composing metamaterials for strong magnetic resonance andnegative refraction. Besides enabling efficient subwavelengthconfinement of spatial modes, compared with other nanostructures, thetop and bottom metal layers of MIM waveguides can be integrated aselectrodes in a straightforward manner in the electro-optic system, bothof which minimize overall device size.

In certain aspects, the present disclosure pertains to improved spectrumfiltering devices generated by using MIM waveguide resonators or otherplasmonic resonator structures based on similar behavior. For example,in certain variations, the present teachings pertain to plasmonic MIMnanoresonator structures capable of filtering white light intoindividual colors (wavelengths or ranges of wavelengths) across theentire visible light band of the electromagnetic spectrum. Furthermore,the principles of the present disclosure can also be applied to otherwavelength ranges.

In this context, surface plasmon-based nanostructures are attractive dueto their small dimensions and the ability to efficiently manipulatelight. In certain aspects, a method for plasmonic optical spectrumfiltering is provided, which comprises filtering an electromagneticspectrum by optical resonance of a plasmonic resonator structurecomprising an electrically conductive metal grating structure adjacentto a dielectric material to generate a filtered output having apredetermined range of wavelengths. In certain aspects, the metalgrating structure is a subwavelength grating (with respect to thefiltered output, for example) that forms a nanoresonator. In accordancewith certain aspects of the present technology, optical resonanceincludes selective conversion between free-space waves and spatiallyconfined modes in plasmonic nanoresonators including subwavelengthgrating (e.g., in metal-insulator-metal stack arrays) to provide wellcontrolled transmission spectra through such arrays by using relativelysimple design rules. Accordingly, the inventive technology pertains toimproved spectrum filtering devices generated by using nanoresonators torealize the photon-plasmon-photon conversion efficiently at specificresonance wavelengths. The present teachings provide high-efficiencyspectrum or color filters capable of transmitting arbitrary colors.These artificial nanostructures provide an approach for high spatialresolution color filtering and spectral imaging with extremely compactdevice architectures.

Thus, in various aspects, the present teachings provide an opticalspectrum filtering device that comprises a filter assembly comprising afirst electrically conductive metal grating structure. By “gratingstructure” it is meant that the metallic material forming the structurecomprises one or more openings there through to permit certainwavelength(s) of light to pass through. For example, in certainpreferred aspects, a grating structure may comprise a plurality of metalrows or discrete regions spaced apart, but substantially parallel to oneanother. The spacing between adjacent rows defines a plurality ofopenings through which certain wavelengths of light may pass. Thegrating may also comprise a second plurality of metal rows having adistinct orientation from the first plurality of rows that are likewisespaced apart, but substantially parallel to one another. The first andsecond plurality of metal rows may intersect or contact one another atone or more locations to form a grid structure. It should be noted thatin preferred aspects, the grating comprises at least two rows to form atleast two openings, but that the number of rows and layers of distinctgrating structures are not limited to only two, but rather may comprisemultiple different designs and layers. Further, as described below,while the adjacent metal rows or other regions of the plurality arepreferably distanced at a sub-wavelength distance from one another (adistance of less than the target wavelength or range of wavelengths),each respective pair of rows may define a distinct distance for eachopening (or slit diameter) there between and thus will permit differentwavelengths of light to travel there through. As discussed in greaterdetail below, a period between a pair of metal rows determinesperiodicity of the openings or slits in the grating structure, whichrelates to a wavelength (or range of wavelengths) generated by thefiltering device. In certain variations, a periodicity defined by atleast two openings of an electrically conductive grating structuredetermines a predetermined range of wavelengths that is transmitted in atransmission-type filtering device or reflected in a reflection typefiltering device. Thus, a periodicity of the openings in the gratingstructure relates to spacing of adjacent metal rows, which determines arange of wavelengths in the filtered output(s) or color(s).

Plasmonic optical spectrum filtering devices having high peaktransmission and high efficiency of a thin-film color filter are ofprimary interest. Depending on the specific applications, in certaintechnologies, simultaneous polarization of the transmitted light isdesirable, while others applications may require narrow spectral peaksfor highly specific wavelength filtering.

In certain aspects, a resonator structure comprises both a firstelectrically conductive metal grating structure and a second distinctelectrically conductive metal grating structure. In certain embodiments,the first and second metal grating structures sandwich an activematerial, such as an insulator or dielectric material. The first andsecond grating structures can be arranged to have a complementaryconfiguration where the gratings are substantially vertically alignedwith one another (to commonly define the same openings). In certainembodiments, the arrangement of the first and second grating structuresin conjunction with the presence of the dielectric material layer(s)forms a filter assembly that is capable of transmitting a portion of anelectromagnetic spectrum through at least one opening defined betweenrespective metal rows of the grating structures, optionally through atleast two openings, to generate a filtered output having a predeterminedrange of wavelengths.

In various aspects, the present disclosure provides methods of formingvarious micro-structural features with various sizes, orientations,shapes, and configurations, especially grating structures for theresonator stack. Various multilayer surfaces can be formed according tothe present disclosure in a variety of patterns (not limited toexemplary layered structures discussed herein). In various aspects, themethods of the disclosure can be used to form wire grating resonators,electrodes, and/or polarizers, having at least one microscale and/ornanoscale feature (such as nano-grating that defines a multi-layergrating structure). Such grating nanostructures can have a variety ofdifferent shapes tailored to the end application; by way of example; asuitable wire grid polarizer has a period (i.e., interval/distancebetween a first feature and a second feature, see FIGS. 2 and 3, forexample) of less than about 1 μm suitable for polarizing and/orfiltering electromagnetic energy waves in the visible (wavelengthranging from about 400 nm to about 800 nm) to near-IR (wavelengthsranging from about 1 μm to about 10 μm). As noted above, in variousaspects, subwavelength grating structures are particularly desirable forthe inventive plasmonic resonator devices, which means that one or moreof the grating dimensions is smaller than a wavelength or range ofwavelengths that are filtered by the device (e.g., a sub-wavelengthgrating means a grating structure for filtering visible light having oneor more dimensions that are less than a wavelength of 1 μm, preferablyless than about 600 nm, depending on the particular wavelength of acolor to be generated). As described in certain embodiments below, wiregrid structures according to the present teachings in the form ofsubwavelength metallic gratings are an attractive alternative toconventional polarizer filters, because they provide a high extinctionratio between the transmitted transverse magnetic (TM) polarized lightand the reflected transverse electric (TE) polarized light over a widewavelength range and incident angle with long-term stability.

The techniques described in this disclosure are generally applicable toa variety of display devices, including any flat panel display havingeither transmission or reflection type, especially for high input powerapplication like a three-dimensional projection displays.Electromagnetic spectrum filters, such as color filters, are animportant component for various display technologies, including flatpanel displays, liquid crystal displays (LCD), projection displays (suchas using digital mirror technology, or liquid crystal on silicon (LCoS),eye-wear displays, complementary metal-oxide-semiconductor (CMOS) imagesensors, IR imagers, light emitting diodes, and the like. For example,transmissive optical spectrum filters are widely utilized inapplications such as liquid crystal display (LCD) panels. The design ofthe thin film structure can significantly simplify the color filter,polarization and liquid crystal control used in current LCD displays,for example. Compared with the aforementioned color-filtering methods,the inventive design significantly improves absolute transmission, passbandwidth and compactness. In addition, the filtered light is naturallypolarized, making it particularly attractive for direct integration inliquid crystal displays (LCDs) without requiring a separate polarizerlayer or filter.

Furthermore, in certain aspects of the present teachings, a method and adevice are disclosed to increase the energy harvesting area of computingdevices by integrating photovoltaic (“PV”) functionality directly intothe display. Hence according to certain variations of the presentteachings, a novel reflective colored display is provided, which isviewable under direct sunlight and can concurrently harvest incidentlight and generate electrical power. Therefore, a method and apparatusare provided to increase the energy harvesting area of computing devicesby integrating PV functionality directly into the display.

The present technology provides resonator structure, such as ametal-dielectric-metal structure, that realizes spectrum filteringfunction for both transmission and reflection types. By tuning thedielectric layer thickness and period of metallic grating structures,the transmission or reflection peak covers a range of predeterminedwavelengths, such as the visible or near-infrared (near-IR) ranges.Particularly suitable visible and infrared electromagnetic radiationincludes, visible light having wavelengths ranging from about 390 toabout 750 nm and infrared radiation (IR) (including near infrared (NIR)ranging from about 0.75 to about 1.4 μm). Filtered electromagneticradiation can have a wavelength in a range of about 625 nm to 740 nm forred; orange is at about 590 nm to about 625 nm; yellow is at about 565nm to about 590 nm; green is at about 520 nm to about 565 nm; blue orcyan is at about 500 nm to about 520 nm; blue or indigo is at about 435nm to about 500 nm; and violet is at about 380 nm to about 435 nm.Further, in certain aspects, the filtered light may be extra-spectral ora mixture of several different wavelengths. For example, magenta is anextra-spectral mixture of red (625 nm to 740 nm) and blue (435 nm to 500nm) wavelengths.

Conventional optical filters using pigment dispersion to produce RGBcolors are manufactured by four separate processes, which not onlycomplicate the manufacturing but also waste many chemical materials inthe process. In a conventional LCD display, separate liquid crystal (LC)alignment layers, polarizer sheets, and transparent electrodes arerequired. As background, an exemplary simplified conventional LCD paneldisplay 20 is shown in FIG. 1. Along the lower bottom side, whichreceives light to be processed, a rear polarizer 22 is adjacent to afirst transparent substrate 24 (e.g., a glass substrate), while alongthe upper top side a front polarizer 32 is adjacent to a secondtransparent substrate 34. On the bottom side, a transparent lowerconductor 40 (or electrode) is disposed adjacent to a lower alignmentlayer 42 (having a surface morphology that induces a predeterminedorientation of liquid crystals upon application of current thereto). Thelower first substrate 24 also has an external electrical contact 44 anda contact bridge 46 that contacts an upper alignment layer 52.

A liquid crystal compartment (60) is formed by a seal 62 in contact withthe first substrate 24 and the upper alignment layer 52 to contain aplurality of liquid crystals 64. Two spacers 66 are also disposed withinthe liquid crystal compartment 60 in contact with both the upper andlower alignment layers 42, 52. Adjacent to the upper alignment layer 52is an upper transparent conductor (or electrode) 50. The upper and loweralignment layers 42, 52 have complementary surface morphologies thatinduce a preferred orientation for the liquid crystals 64 when voltageor electrical potential is applied to permit light to transmit androtate through the liquid crystals. On the upper side of the LCDassembly 20 adjacent to the upper transparent conductor 50 is a colorfilter 70 disposed within a black absorptive matrix 72, which isadjacent to the upper substrate 34. Thus, when electrical potential isapplied to the upper and lower conductors, the liquid crystals areoriented such that white light (generated within the display device) ispermitted to pass through the rear polarizer 22, into the liquidcrystals 64, through the color filter 70, which is then transmitted outof the front polarizer 32 to provide a filtered colored light. In theabsence of electrical potential applied to the conductors/electrodes 30,40, the liquid crystals 64 are randomly oriented and no incident lightpasses through the liquid crystal compartment. LCD pixels frequentlyhave three adjacent color filter assemblies (tuned to provide one of ared-green-blue “ROB” color, for example) that can be selectivelyactivated. Typical transmission efficiencies for such conventional LCDfiltering devices are on the order of only about 6%, while the cost ofsuch conventional color filter assembly accounts for about 20% of thetotal cost of an LCD panel.

In certain aspects, a first grating structure (e.g., grid or meshpattern) is formed on a major surface of the substrate having a firstorientation. In one example, such as that shown in FIGS. 2 and 3, agrating structure pattern 200 is formed over a substrate 202 having amajor surface 204. The grating pattern may be formed of a conductivematerial, such as a thin metallic film. A plurality of substantiallyparallel rows 208 of such thin film conductive materials are formed onmajor surface 204. The plurality of rows 208 in the grating pattern 200optionally comprises an electrically conductive metal, such as gold oraluminum. In certain variations, gold is a preferred electricallyconductive metal for rows 208.

The grid or grating pattern 200 of metal rows formed on substrate 202defines a period “p” (a distance defined from a first side 212 of afirst row 214 to a first side 216 of a second adjacent row 218). Adistance “d” between adjacent rows 208 is considered an opening (oraperture or slit) as described below. It should be noted that distance“d” may vary through the grating pattern 200 where d is represented byd=p−a. A metal row 208 has a height “h” and a width of each metal row208 is “a.” A duty cycle is defined by f=a/p. Periodicity refers to atleast one period (p) between a pair of rows in the grating pattern, butwhere there are more than two openings typically to a repeating period(p) in the grating pattern. In one exemplary embodiment, the period (p)between rows 208 is about 700 nm, the width (a) of each row 208 is about70 nm, and the height (h) of each metal row 208 is about 40 nm. Thus, ahigh transparency resonator structure can be designed by adjusting metalrow width (a) and period (p) so that different wavelengths of light canbe transmitted through openings (d). High conductance can likewise beachieved by adjusting the thickness (h) of the film of metal materialforming rows 208. Such a grating pattern 200 provides a highly flexibledesign that can be readily tailored for different performance criteria.

As shown in FIG. 3, a filtering device 220 comprises an assembly ofmultiple thin layers (230, 240, 250) disposed on a substrate 222 havinga major surface 224. A first plurality of rows of conductive metalmaterial 228 are disposed on substrate 222 and define an initial patternor grid on major surface 224 to define a first metal grating layer 230.The first plurality of rows 228 has a height designated “h₁.” An activematerial, like an insulator or dielectric material 232, is disposed overthe first metal grating layer 230 (over the plurality of rows ofconductive metal material 228) to define a second layer 240. Thedielectric material 232 has a height designated “h₂.” Finally, a secondplurality of rows of conductive metal material 234 is disposed ondielectric material 232 to define a second distinct grating structure orthird layer 250. The second plurality of rows 228 has a heightdesignated “h₃.” The second plurality of rows of conductive metalmaterial 234 are complementary to the first plurality of rows ofconductive material 228, as they are substantially vertically andhorizontally aligned and define similar widths “a” on major surface 224.Together, the first plurality of rows of conductive material 228 andsecond plurality of rows of conductive material 234 sandwich dielectricmaterial 232 to form a vertical microstructure 248 that creates themulti-layered assembly resonator structure. Vertical microstructure 248has a height of “h” measured from major surface 224 where h=h₁+h₂+h₃ anddefines a grating pattern or grid on major surface 224. Therefore,subwavelength openings or slits 252 are formed between the respectivevertical microstructures 248 and have a diameter “d” represented byd=p−a, wherein p is the period between adjacent microstructure rows 248and a is the diameter of the microstructure rows 248.

In certain variations, an optical spectrum filtering device generates afiltered output that exits the filter device having a predeterminedrange of wavelengths in the visible light range. For certainembodiments, color filter designs prepared in accordance with thepresent teachings optionally produce cyan, magenta, and yellow (CMY)colors (see for example, FIGS. 6A-6C), which can be used to form apixel. In other embodiments, such as the one shown in FIGS. 10A-6D, red,green, and blue (RGB) as well as yellow primary colors can be formed.The color scheme is additive and thereby can generate arbitrary colors.

In certain variations, the optical spectrum filtering devicereflection-type filter generates a filtered output that exits the filterdevice having a predetermined range of wavelengths in the visible lightrange. Such a predetermined range of wavelengths may include a colorselected from the group consisting of: red, green, blue, cyan, magenta,yellow, and combinations thereof, by way of non-limiting example.

The present teachings pertain to constructing novel image pixels fordisplays with distinct colors by incorporating a plasmonic resonatorstructure. The colors are easily controlled by changing structuralparameters. For example, a transmission spectrum for an exemplary colorpixel can exhibit an efficiency of greater than or equal to about 60%efficiency, optionally of greater than or equal to about 65% efficiency,optionally greater than or equal to about 70% efficiency, optionallygreater than or equal to about 75% efficiency, optionally greater thanor equal to about 80% efficiency, optionally greater than or equal toabout 85% efficiency, and in certain preferred aspects, optionallygreater than or equal to about 90% efficiency which is desirably high ascompared to a conventional color pixel and similar to predictedefficiency based on simulation results. Furthermore, efficiency of afiltering device for a pixel incorporating a plasmonic resonatorstructure can be significantly increased.

For example, in a narrow band color filter design shown in FIG. 25, abuffer layer is disposed between a metal grating comprising aluminum anda high refractive index dielectric material. A variety of buffermaterials can be selected, including poly(3,4-ethylenedioxythiophene)(PEDOT) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS), cesium carbonate (Cs₂CO₃), silicon dioxide (SiO₂), zincoxide (ZnO), vanadium pentoxide (V₂O₅), nickel oxide (Ni₂O), Molybdenumoxide (MoO₃), and combinations thereof.

In the embodiment of FIG. 25, a low refractive index material layer isselected as a buffer and thus comprises a low refractive indexdielectric material (such as SiO₂, where n=1.5). The low index layer hasa thickness of about 50 nm and serves as a buffer layer. A highrefractive index dielectric material (Si₃N₄) (refractive index, n=2.0)is provided adjacent to the buffer layer at a thickness of about 100 nmand serves as a modulation layer. The high index dielectric materialmodulation layer is disposed adjacent to a glass substrate comprisingSiO₂. Inclusion of such a buffer layer can significantly increasetransmission efficiency.

A periodicity of the grating structure is variable, so at one end, anopening between a pair of aluminum rows is greater than at a secondopposite end. Thus, in the structure shown in FIG. 25A, a first periodis 300 nm, a second period is 250 nm, and a third period is 450 nm. FIG.25B shows experimental filtering results for different discrete regionscorresponding to the distinct periods, where the filtered output is bluein a first region, green in a second region, and red in a third region.FIG. 25C shows a simulation for such a device structure shows wavelengthversus transmission for blue, red, and green filtered light outputs,while comparative actual experimental results are shown in FIG. 25D(including the buffered layer structure), where transmission levels aredesirably high for blue, red, and green as compared to the simulatedresults and a full width at half maximum (FWHM) ranges from about 25 to30 nm. Such filter devices have desirably high transmission and thushave significantly improved energy efficiency.

As discussed above, subwavelength metallic gratings according to certainaspects of the present disclosure are thin and planar structures and maybe easily integrated with other thin-film optical elements. For example,bilayer metal wire grids can be considered as two metal gratingsseparated by a certain distance (for example, separated by thedielectric or insulator material). Not only does this type of multiplelayer structure show a very high extinction ratio between the lights oftwo orthogonal polarizations, but it also offers the advantage of easyfabrication and defect tolerance.

As shown in FIG. 11, the present disclosure provides a plasmonicnanoresonator filter device 600 for color filtering. A portion of alight source 630 is transmitted through the filter device 600 andgenerates a filtered output 640. The structure of the filter device 600is as follows. A transparent substrate 602 defines a major surface 604on which a plurality of rows 620 is formed. A resonator assemblystructure 622 comprises a first electrically conductive metal gratingstructure 606, such as a subwavelength nanograting. The firstelectrically conductive metal grating structure comprises at least twoopenings 624 capable of transmitting a portion of an electromagneticspectrum so as to generate a filtered output 640 having a predeterminedrange of wavelengths via an optical resonance mechanism. Furthermore,the openings 624 in the first electrically conductive metal gratingstructure 606 have at least one dimension that is less than thepredetermined range of wavelengths for the filtered output. Theresonator assembly 622 also comprises a dielectric material layer 608.The resonator structure 622 optionally further comprises a secondelectrically conductive metal element disposed along an opposite side ofthe dielectric material 610.

In the embodiment shown in FIG. 11, the second electrically conductiveelement is in the form of a second electrically conductive metal gratingstructure 610 disposed over dielectric material layer 608. The firstelectrically conductive metal grating structure 606, the dielectricmaterial 608, and the second electrically conductive metal gratingstructure 610 are arranged to be substantially aligned in a horizontaland vertical orientation and thus together define the openings 624through the resonator structure assembly 622 that generates the filteredoutput having the predetermined range of wavelengths. A periodicitydefined by resonator assembly 620 (e.g., see period “p” which can becalculated by a+d of openings 624 of the electrically conductive gratingstructures 606 and 610) determines the predetermined range ofwavelengths that is transmitted through the plasmonic filtering device600.

In certain variations, the first electrically conductive metal gratingstructure 606 and the second electrically conductive metal gratingstructure 610 may comprise a metal selected from the group consistingof: gold, aluminum, silver, copper, and combinations thereof. In yetother variations, the dielectric material 608 can be selected from thegroup consisting of: silicon nitride (Si₃N₄), zinc selenide (ZnSe), zincoxide (ZnO), zirconium oxide (ZrO₂), and titanium oxide (TiO₂). Thetransparent substrate 602 can be formed from conventional transparentsubstrates for optical devices, such as silicon dioxide (SiO₂) ormagnesium fluoride (MgF₂).

One suitable optical plasmonic spectrum filtering device having a designlike that in FIG. 11 thus has a subwavelength periodic MIM stack arrayor resonator structure assembly 622 on a magnesium fluoride (MgF₂)transparent film 602 with period (p). For each stack (each row 620), a100 nm-thick zinc selenide (ZnSe) dielectric layer 606 is sandwiched bytwo 40 nm-thick aluminum (Al) layers (606, 610), where the thickness ofthe dielectric core is determined on the basis of the spatial extensionof SP waves inside the dielectric ZnSe layer 608 at the visiblefrequencies. The 100 nm-thick ZnSe dielectric layer 608 ensures theefficient coupling of SP modes at the top and bottom edges of the stackor assembly 622, whereas the 40 nm-thick Al layer 606 prohibits thedirect transmission of the incident light. The duty cycle of the stackarray is about 0.7. The bottom Al grating 606 is used to coupleselectively the incident light into plasmon waveguide modes bydiffraction, whereas the top Al grating 610 efficiently reconverts theconfined plasmons to propagating waves by scattering and transmits thelight 630 to the far field in the forward direction.

FIG. 10A presents a schematic diagram of an embodiment of inventivenanoresonators like that in FIG. 11, but generating four distinct colors(red, green, blue, and yellow). For TM-polarized waves (the E-field isperpendicular to the Al grating direction), the transverse magneticplasmon dispersion relation is plotted in FIG. 10B and the MIM structureis given by

$\begin{matrix}{{{ɛ_{1}k_{z\; 2}} + {ɛ_{2}k_{z\; 1}{\tanh\left( \frac{{- {ik}_{z\; 1}}d}{2} \right)}}} = 0} & (1)\end{matrix}$for the antisymmetric mode, and by

$\begin{matrix}{{{ɛ_{1}k_{z\; 2}} + {ɛ_{2}k_{z\; 1}{\coth\left( \frac{{- {ik}_{z\; 1}}d}{2} \right)}}} = 0} & (2)\end{matrix}$for the symmetric mode; kz is defined by the momentum conservation:

$\begin{matrix}{k_{{z\; 1},2}^{2} = {{ɛ_{1,2}\left( \frac{\omega}{c} \right)} - k_{x}^{2}}} & (3)\end{matrix}$where ∈ is the complex dielectric constant, subscripts 1 and 2 theinsulator and metal layer, and d the thickness of the insulator layer.Here, all materials are assumed to be non-magnetic so that the magneticpermeability is equal to 1. The Al/ZnSe/Al stack array has a 0.7 dutycycle and therefore the dielectric constant of the insulator layer iscalculated following the effective medium theory. The complex dielectricconstant of Al is described by fitting the experimentally measured data.The spatial extension of SP waves in the ZnSe layer is calculated to beabout 100 nm at visible frequencies. Therefore, a 100 nm-thick ZnSelayer is selected to ensure the efficient coupling of SP modes at thetop and bottom edges of the stack. Thereafter, the plasmon dispersionsfor the antisymmetric and symmetric modes are obtained by solvingequations (1) and (2).

For TM-polarized waves (the E-field is perpendicular to the Al gratingdirection), the transverse magnetic plasmon dispersion of the Al/ZnSe/Alstack array is plotted in FIG. 10B. Here only normal incidence isconsidered and therefore the stack period is related to the plasmontransverse wave vector as P=27 r/k, by the ±1st order diffraction. FromFIG. 10B, it can be clearly seen that the SP antisymmetric mode has anear-linear dispersion across the whole visible range. Therefore, the SPantisymmetric modes can be used as intermediates to couple the incidentplane wave selectively in and scatter the confined SP modes out to thefar field. The close-to-linear dispersion makes design of the inventivefilters straightforward for any colors across the entire visible band.As an example, the red, green and blue spots in FIG. 10B represent thethree primary RGB colors. They have different transverse wave vectorsthat correspond to specific stack periods by the ±1 st orderdiffraction. The simulated transmission spectra for RGB colors are shownin FIG. 10C. The corresponding period of the stack is 360, 270 and 230nm that can be fabricated by micro and nanofabrication technologies.However, the TE-polarized light (the E-field is parallel to the Al wiredirection) does not support the excitation of SP modes and thus there isno obvious light conversion process. As a result, the TE-polarized lightis strongly suppressed at resonance wavelengths and the transmissionsare extremely low. This indicates that such transmission color filterscan simultaneously function as polarizing elements, a highly desirablefeature for display applications.

FIG. 10D shows the simulated time-average magnetic field intensity andelectric displacement (arrow) profiles in one MIM stack corresponding tothe red spot in FIG. 1 b. The TM-polarized incident light has awavelength of 650 nm and the stack period is 360 nm. The magnetic fieldintensity shows that most of the incident light is coupled intoantisymmetric waveguide modes with maximum intensity near the edges oftop and bottom Al gratings, which supports the principles upon which theinventive designs are based. From the electric displacement, efficientcoupling of incident light to the SP antisymmetric modes is realized bythe strong magnetic response (as previously observed where the electricdisplacement forms a loop and results in strong magnetic fields opposingthat of the incident light inside the dielectric layer).

Thus, in certain variations, the inventive plasmonic optical spectrumfiltering devices serve as a polarizer device. Further, such inventiveplasmonic optical spectrum filtering devices can also form a pixel for adisplay device, where a plurality of filtered colors can be formed inclose proximity to one another. In other embodiments, a first conductivemetal grating is made on top of one or two dielectric layers ofdifferent refractive indices to perform the filter function, offeringsuch utility as high transmission and narrow bandwidth (see for example,FIG. 25). In other embodiments, a two-dimensional (2D) grid structurecan be used to reduce the polarization dependence.

The control of light interaction with nanostructures and their uniqueapplications in photonics can be used in development of nanofabricationand characterization techniques for light management. As discussedabove, optical resonance effects in nanoholes, nanoslits, and relatednanostructures can be used for color filter applications. According tocertain aspects of the present teachings, a resonator structure can be afiltering assembly comprising multiple layers, including a first metalgrating and an active material, which can be selected from a dielectricmaterial or a photoactive material. A metal-dielectric-metal buildingblock suitable for use as a color filter in accordance with certainembodiments of the present disclosure is similar to an organic solarcell or photovoltaic structure (in which the organic semiconductormaterials are sandwiched by two conductive electrodes). In suchembodiments, a photoactive material comprising an organic semiconductoris incorporated into the multilayered film assembly as an integral partof a specially designed photonic color filter for energy conversion. Incertain embodiments, a photoactive material may comprise a bulkheterojunction material comprising both electron donor and electronacceptor materials commonly used in photovoltaic applications. When thelight extinction occurs at the specific wavelength, light energy isabsorbed by the organic semiconductor photoactive material, whichproduces photocurrent. Therefore, in accordance with the presentteachings, in certain embodiments the devices have a design that is notonly able to filter specific colors, but also generate photocurrent andthus electrical energy as a solar or photovoltaic cell.

In certain embodiments, a reflectance type color filtering device iscapable of power generation. Reflective colors are brilliant and free ofglare in sunlight. They are superior to transmissive filter technologiesfor outdoor usage, and even better under direct sunlight for more energyabsorption. Therefore, in accordance with certain embodiments of thepresent disclosure, certain optical spectrum filtering devices arecapable of recycling energy otherwise wasted in most of displayapplications (that require color filters to achieve desirable colors).Furthermore, organic photovoltaic (OPV) cell structures are incorporatedinto certain dual-function device embodiments. Therefore, the approachalso takes the advantages of the OPV, such as low cost, easyfabrication, and compatibility with flexible substrates over a largearea. In addition, alternative applications of OPVs are contemplatedwhich complement the great efforts in improving power conversionefficiency (PCE) and practical fabrication methods.

In one exemplary embodiment shown in FIG. 4, a multi-layereddual-function photovoltaic-color filter device 300 is capable ofproducing desirable reflection colors and simultaneously convertingabsorbed light 320 to generate electricity. More specifically, areflective spectra of pixels are made with device 300. Because thereflectance type color filters act similarly to the color paint, e.g.,absorbing light corresponding to specific wavelengths, but reflectingthe others, in the embodiment, the CMY color scheme is employed wherecyan, magenta and yellow are three primary colors. A transparentsubstrate 302 may comprise glass, such as silicon dioxide (e.g.,silica). A first electrode 310 is disposed in a stacking arrangementnear substrate 302 and as shown in FIG. 4 is disposed within a portionof an optional buffer layer 304. For example, a buffer layer 304 can bedisposed over the transparent substrate 302. The buffer layer 304 servesto enhance charge transport from the first electrode 310. The firstelectrode 310 may be an anode and may comprise gold or other conductivematerials. The buffer layer 304 may comprisepoly(3,4-ethylenedioxythiophene) (PEDOT) or alternatively,poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), forexample. Other suitable buffer materials include cesium carbonate(Cs₂CO₃), silicon dioxide (SiO₂), zinc oxide (ZnO), vanadium pentoxide(V₂O₅), nickel oxide (Ni₂O), Molybdenum oxide (MoO₃), and combinationsthereof.

The multi-layered device 300 may comprise one or more photoactiveorganic semiconductor materials 312. The semiconductor layer 312 isadjacent to buffer layer 304. The photoactive semiconductor materiallayer 312 comprises a material like an organic electron donor, such aspoly(3-hexylthiophene) (P3HT), combined with an organic electronacceptor, such as [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) toform a combined layer of mixed P3HT:PCPM. While not limiting the presentdisclosure to any particular organic material system, for purposes ofillustration, a conjugate polymer system of poly(3-hexylthiophene)(P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) shows a bulkheterojunction (BHJ) P3HT:PCBM blend used as an exemplary model system.

A second electrode 314 is disposed along the surface of the photoactivematerial layer 312. The second electrode 314 may be a transparentcathode and may comprise aluminum or other conductive materials. Thesecond electrode 310 comprising aluminum (a cathode) can be in the formof a continuous film that sandwiches the active organic semiconductorslayers (buffer layer 304 and photoactive material layer 312). In certainvariations, either the first electrode or the second electrode may be ananograting. Diameter “D” of the first anode electrode 310 is shown inFIG. 4, as is the period “P” of the structure. FIG. 5B compares theabsorption behavior of the same pixels (FIG. 5C) for a structure like inFIG. 4. It should be emphasized that the color selective property of thestructure shown in FIG. 5A is due to the total absorption (close to100%) of incident light 320 at the specific wavelength range without useany antireflection layer, which reflect complementary colors.

In variations where the resonator structure can serve as a photovoltaicdevice, the display device can also be used as a “touch screen” displaydevice that is capable of detecting a reduced photovoltaic outputindicative of a shadow or positioning of a user's finger adjacent ornear the screen. This indication of reduced photovoltaic output can beused as a user interface for receiving a user input or selection (suchas a “touch screen” input). However, it should be appreciated thatactual contact of the screen may not be necessary in some embodiments.

Thus, FIGS. 6A-6C present the schematic diagram of certainenergy-harvesting color filters prepared in accordance with the presentdisclosure, where the conjugated polymer layers comprise a buffer layer402 of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS) and a photoactive material 410 ofpoly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester(P3HT:PCBM) blend are sandwiched by a nanograting layer 412 comprisinggold and a continuous thick film forming cathode 414 comprisingaluminum. The nanograting 412 is adjacent to a transparent substrate 404comprising glass. Each of the filter devices in FIGS. 6A-6C havedifferent periodicity (FIGS. 6G-6I) that generates different colors asfiltered output, as can be seen in FIGS. 6D-6F). The selection of eachmaterial and its role will be further explained below.

FIGS. 6G-6I shows the scanning electron microscope (SEM) images of thefabricated nanogratings. Here, gold (Au) is chosen for the nanogratings412 due to its excellent conductivity and appropriate work function asan anode. The periodic Au nanograting structures 412 are fabricatedusing nanoimprint lithography (NIL), followed by a reactive ion etching(RIE), metal deposition and lift-off process, to produce large areatransparent electrode onto which OPV cells can be easily fabricated. Thefabrication of the complete OPV structures using those Au nanogratings412 as an anode is as follows. First, a conductive PEDOT:PSS layer,often used in OPV structures as a hole transporting layer (and with thehigh work function for hole collecting) is cast on the Au nanogratinganode (as a “buffer” layer 402). Such “composite” anode structurescomprising metallic nanogratings 412 and a buffer layer 402 (e.g.,comprising PEDOT:PSS) can ensure efficient hole collection and transportbetween the metal rows/lines.

Next, a high performance bulk-heterojunction (BHJ) photoactive layer 410comprising a blend or mixture of P3HT and PCBM is constructed, servingto convert absorbed light to photocurrent (see electrical connectors 422in FIG. 6B showing an exemplary current collection device). Afterthermal annealing to optimize the BHJ nanostructures in the photoactivematerial layer 410, a continuous Al film layer is thermally deposited asa cathode 414. Al is selected due to its excellent performance as acathode material for the OPV cells when combined with a thin LiF layerand cost-effectiveness. An ultrathin 1 nm thick LiF film (not shown) isoptionally used to improve the performance of OPV cells, but does notappear to affect the optical properties of the device. The Al layer 414also prohibits the direct transmission of the incident light.

Following the inventive design principles and the fabrication processesdescribed above, a filtering device for generating three distinctfiltered outputs in primary CMY colors (cyan, magenta and yellow) undernatural light conditions is created. First, bulk heterojunction (BHJ)active layers 410 of three thicknesses (90 nm, 65 nm and 50 nm) areformed to control the TE polarized light to generate CMY colors,respectively. Then nanogratings 412 comprising gold of threecorresponding periods (420 nm, 280 nm and 220 nm) are selected tocontrol the TM polarized light under the given photoactive layerthickness. In all three cases, the thicknesses of the Au nanogratings412 and the buffer (PEDOT:PSS) layer 402 are 40 nm and 30 nm,respectively. All the Au nanogratings 412 have about 0.7 duty cycle (Auline width is 0.7 of the period). A light 420 is illuminated onto thecolor filter from the Au anode 412 side and the photograph images aretaken showing distinct reflected CMY colors (filtered outputs 416C,416M, 416Y over large areas (FIGS. 1D-1F).

Large area metallic nanostructures can be used as semi-transparentelectrodes for organic optoelectronics by controlling their opticaltransparency and electrical conductivity. Advantageously, the inventivedesign provides periodic metallic nanogratings in the multi-layereddevice, which act not only as nanostructures to modulate the incidentlight to generate different colors, but also as an optionalsemi-transparent anode for an organic photovoltaic (OPV) cell. In suchstructures, a low work function conductive metal film, like aluminum,can act as a cathode.

Color filters are desired to be polarization insensitive to the naturalincident light. To achieve this, different design strategies can beemployed for transverse electric (TE) and transverse magnetic (TM)waves, so that similar reflection spectra for both polarizations can beobtained for each color. First in certain aspects, the structureresembles a Fabry-Perot cavity. When the light impinges on thenanogratings (e.g., comprising gold), it will interfere with the wavesreflected from the bottom metal element or film (e.g., a film comprisingaluminum). When destructive interference occurs, the reflection spectrawill reach a minimum, and the light energy of the corresponding colorwill be absorbed and complementary color reflected by the structure.

FIGS. 7A and 7B show the calculated reflection spectra maps for TEpolarized light (the E-field is parallel to the Au nanogratingdirection) as a function of P3HT:PCBM blend layer thickness (FIG. 7A, Augrating period fixed at 280 nm) and as a function of Au nanogratingperiod (FIG. 7B, blend layer thickness fixed at 50 nm), respectively.The blue regions in these maps represent the reflection minima where thelight energy is absorbed by the structure. As expected from theFabry-Perot interference, the resonant absorption wavelength is linearlyproportional to thickness of a blend layer but almost independent of thenanograting period. For TM waves (the E-field is perpendicular to the Aunanograting direction), additional consideration needs to be taken, asit is well known that TM waves can be efficiently coupled to the plasmonmodes through the subwavelength grating structures.

FIG. 7C gives a calculated map of reflection spectrum for TM polarizedwaves as a function of Au nanograting period. Here the P3HT:PCBM blendlayer thickness remains 50 nm. In addition to the absorption bandproduced by the Fabry-Perot interference, there are two more resonantabsorption stripes (marked with white dashed lines) that originate fromthe splitting of SP modes in the plasmonic waveguide structures. Thedispersion characteristics of these modes show strong dependence on theAu nanograting period because it provides the phase matching conditionfor the TM polarized light to couple to the SP mode. Taking advantage ofthe insensitivity of Fabry-Perot resonance to the Au nanograting periodand choosing a particular period so that the TM polarized light isabsorbed around the same wavelength as the TE polarized light, producespolarization independent reflection colors.

FIG. 7D shows simulated magnetic field intensity for TM and TE waves atthe same resonant absorption wavelength of 490 nm. The thickness ofP3HT:PCBM blend layer and the periodicity of Au nanogratings are 50 nmand 220 nm, respectively. It can be clearly seen that the fielddistribution for TM polarization exhibits plasmon behaviors at theAu-PEDOT:PSS and Al-P3HT:PCBM interfaces. For the TE polarization, thefield distribution resembles that of conventional Fabry-Perotinterference, supporting the inventive design principles.

FIGS. 8A-8F show reflection spectra of the cyan, magenta, and yellow(CMY) colors of the device structure: simulation results by rigorouscoupled wave analysis (RCWA) method (upper panel—FIGS. 8A-8C), andexperimental results measured by using a broadband white light source(lower panel—FIGS. 8D-8F). As can be seen, the expected color filteringbehavior is obtained, and the experimentally measured spectra arewell-matched with the simulation results calculated for both polarizedand unpolarized light conditions.

Finally photovoltaic properties of the reflective color filter devicesare measured under amplitude modification (AM) 1.5 G simulated sunlight(at 100 mW cm⁻² intensity) and the current density versus voltagecharacteristics are summarized in FIG. 9. The measured power conversionefficiency (PCEs) for the CMY color filters are 1.55%, 0.82%, and 0.60%,respectively. The cyan-colored devices are those with the thickestphotoactive layer (thickness of about 90 nm), leading to sufficientlight absorption in this structure, showed the best efficiency with thehighest short circuit current density (k) and fill factor. Themagenta-colored devices have well-matched absorption near the maximumenergy band of electron-donor materials P3HT, usually centered around550 nm have the comparable J_(sc) with the cyan-colored, even with thethinner photoactive layer (approximately 65 nm thickness).

The yellow-colored filters also successfully generate electrical poweras OPV cells, even though they have reduced absorption around the energyband of electron-donor with the thinnest photoactive layer (about 50 nm)giving the lowest J_(sc). Furthermore, because the metallic nanogratingsof the present teachings simultaneously serve as the semi-transparentelectrode for the solar cell, the use of wider metal lines (0.7 dutycycle) is highly desirable because this significantly reduces theresistance of the electrode, which is important for large area solarcells without degrading the power conversion efficiency (PCE). As anexample, current OPV cells built on indium tin oxide (ITO) transparentelectrodes still suffer from the insufficient conductance of thetransparent ITO when they are made large area due to the voltage drop onthe resistive electrode. Moreover, the superior flexibility of metallicnanostructures on the flexible substrate without conductance degradationunder bent condition and the applicability to large area roll-to-rollprocessing make these dual-function devices useful to large areaflexible display applications.

Another advantageous benefit is that the longitudinal thickness of suchan inventive photonic color filter is less than 200 nm, which is about 2orders of magnitude thinner than that of traditional colorant basedfilters (for LCD display panels). This is very attractive for the designof ultrathin colored devices.

Such reflective color filtering embodiments simultaneously integrate theOPV function into a single device. The absorbed light by the colorfilter, which is otherwise totally wasted, is harvested by the OPV togenerate photocurrents. The dual function devices of the presentteachings, which is expected provide far superior and higher efficiencyin display devices and is likely to lead to significantly improvedenergy-efficient e-media.

Example 1

A gold nanograting is fabricated as follows: Three types of large areaAu nanogratings having different periodicity (420 nm, 280 nm and 220 nm)are fabricated by nano-imprint lithography (NIL-based) processes. NIL isperformed in Nanonex NX2000 nanoimprinter (Princeton, N.J.) using a SiO₂mold with 0.7 duty cycle on a MRI-8030 resist (Microresist TechnologyGmbH) spin-casted on glass substrates, at a pressure of 600 psi and atemperature of 180° C. for 5 minutes. After cooling and demolding, Ti isselectively deposited on each sidewall of the imprinted gratingstructures by angled deposition. Ti deposited on the resist patternsinduced the undercut structures during O₂ reactive ion etching (RIE),facilitating the lift-off process. O₂ RIE (20 sccm 0₂, 12 mTorr chamberpressure, and 30 W bias power), deposition of 40 nm Au with 1 nm Tiusing electron-beam evaporator and lift-off process completed thefabrication of Au nanograting structures on a substrate.

A dual-function device is prepared as follows: Au nanogratings on glassare cleaned in acetone and isopropyl alcohol (IPA) under sonication for20 minutes, respectively, and treated by O₂ plasma for 60 seconds.Cleaned substrates are then transferred to a N₂ purged glove box and thefiltered PEDOT:PSS (H.C. Starck, Clevios PH 500) is spin-casted onto theAu nanograting electrodes to deposit ˜30 nm thick layer which issubsequently baked at 115° C. for 15 min. For the photoactive layer,P3HT (Rieke Metals Inc., 4002-E, ˜91% regioregularity) and PCBM(American Dye Source, Purity: >99.5%) are used as received, and blendsolutions are prepared by dissolving both components in chlorobenzenewith 1:1 ratio by weight. The solution is stirred for approximately 12hours in the N₂ purged glove box to give a homogeneous blend system andfiltered using a 0.45 μm filter. The blend solution is spin-casted ontothe PEDOT:PSS layer, and annealed at 130° C. for 20 min. The thicknessof the blend film is controlled by changing the concentration ofsolution and the spin-coating speed. The thickness of organic layer ismeasured by Dektek profiler. After thermal treatment, LiF (1 nm) and Al(75 nm) are deposited by thermal evaporator at pressure of 8×10⁻⁷ mbarthrough circular-shaped shadow masks.

Solar cell/photovoltaic performance measurements, such as current versusvoltage characteristics, are measured with Keithley 2400 system byilluminating the OPV cells with AM 1.5 G simulated sun light using OrielSolar Simulator with a irradiation intensity of 100 mW cm⁻², which iscalibrated by power meter (OPHIR, Nova-Oriel) and a reference siliconsolar cell.

Example 2

Plasmonic nanoresonators devices for color filtering, such as that inFIG. 10A discussed above, are formed as follows. For easy fabrication,the device is designed as a subwavelength periodic MIM stack array on amagnesium fluoride (MgF₂) transparent film with period P. A glasssubstrate is prepared using a cleaning process. A 220 nm magnesiumfluoride (MgF₂) layer, 40 nm Al, 100 nm zinc selenide (ZnSe) and another40 nm Al are deposited sequentially using an electron-beam evaporatorwith a deposition rate of 0.5 nms⁻¹. To prevent the Al from oxidizing,the samples are stored in an oxygen-free glove box before measurement.For each stack, a 100 nm-thick zinc selenide (ZnSe) dielectric layer issandwiched by two 40 nm-thick aluminum (Al) grating layers. The dutycycle of the stack array is about 0.7.

Patterns are milled with the focus ion beam equipped in a Nova NanoLabDuaLbeam workstation (FEI). Milling is controlled with an AutoFIBprogram, provided by FEI, capable of controlling all parameters relatingto ion and electron beams. The accelerating voltage is 30 kV and currentvaries from 50 to 100 pA, determined by the feature size. Magnificationis from ×6,500 to ×10,000 for patterning. Transmission spectra aremeasured with an inverted microscope (Nikon TE 300; Nikon), and alloptical images are taken with a camera (Nikon D3000; Nikon) mounted onthe front port of the microscope.

FIG. 12A shows the optical microscopy images of the seven square-shapedplasmonic color filters illuminated by a white light. The filters arefabricated using focus ion beam milling of a deposited Al/ZnSe/Al stackon an MgF₂ substrate. The color filters have the stack period changingfrom 200 to 360 nm, corresponding to the color from violet to red. Allthe filters have the same area dimension of about 10 μm×10 μm. Themeasured transmission spectra of RGB filters are given in FIG. 12B,which agrees well with the simulation results shown in FIG. 10D. For TMillumination, stack arrays show the expected filtering behavior withabsolute transmission over 50% around the resonant wavelengths, which isseveral orders of magnitude higher than those of previously reported MIMresonators. This transmission is comparable with the prevailingcolorant-based filter used in an LCD panel, but the thickness of theplasmonic device is 1-2 orders of magnitude thinner than that of thecolorant-based filters. The relative lower transmission for blue coloris due to the stronger absorption loss of ZnSe in the shorter wavelengthrange. The full-width at half maximum of the passbands is about 100 nmfor all three colors.

However, the devices strongly reflect TE-polarized light, as inwire-grid polarizers. Therefore, the transmission of TE-polarized lightis suppressed as shown in FIG. 12B. This feature indicates that thestructure itself can have the roles of color filter and polarizersimultaneously, which greatly benefits LCD technology by eliminating theneed of a separate polarizer layer. In addition in certain embodiments,due to the conductive nature of the Al grating, a separate transparentconductive oxide layer used in LCD module is not necessary.

Besides the standard square color filters, different nanoresonatorarrays can form arbitrary colored patterns on a micrometer scale. By wayof example, a yellow character “M” in a navy blue background is shown inFIG. 12C. The pattern size of the “M” logo measures only 20 μm×12 μm anduses two periods: 310 nm for the yellow letter M and 220 nm for the navyblue background. The optical microscopy image of the pattern illuminatedwith the white light is shown in FIG. 12D. A clear-cut yellow “M”sharply contrasts the navy blue background. It is important to note thatthe two distinct colors are well preserved even at the sharp corners andboundaries of the two different patterns, which demonstrates that theinventive color filter scheme can be extended to ultrahigh resolutioncolor displays.

Simulation and experiments are conducted to investigate the relationshipbetween the opening or slit number and the filtering effect to betterdetermine the relationship between the quantity of openings or slits asit relates to color effects. The inset of FIG. 13 shows the microscopeimages of the samples with 2, 4 and 6 slits (from the bottom to the top)for green and red color filters. The center-to-center distance betweenthe neighboring slits for green and red filters is 270 and 360 nm,respectively, with slit widths of about 80 and 100 nm. Surprisingly,even the structures with only two slits already exhibit distinct colors.With more slits, the green and red colors become better defined and muchbrighter, and agree well with the simulation results shown in FIG. 13.This interesting behavior can be explained by the interference of the SPwaveguide modes. When the incident light is coupled into the SP modesthrough the bottom slits, the counterpropagating SP waves inside theinsulator layer from different slits interferes along the waveguidedirection. Transmission will be enhanced for constructive interferenceat the top slits, and suppressed for destructive interference.

Fundamentally, the filtering effect of the stack array with infiniteslits can also be ascribed to the multiple interferences of SP wavesfrom each slit. The main difference between structures having a fewslits and infinite slits is that the latter can efficiently couple theincident light into SP antisymmetric modes through momentum matching bydiffraction, and therefore the efficiency is much higher. The aboveanalysis and experimental results indicate that very compact structureswith just a few openings or slits can still adequately perform thedesired color filtering function.

Example 3

Plasmonic nanoresonators for spectral and polarimetric imaging areinvestigated and shown in FIGS. 14A-D. By gradually changing the periodsof the plasmonic nanoresonator array, a design for a plasmonicspectroscope for spectral imaging is provided. FIG. 14A is a scanningelectron microscopy (SEM) image of the fabricated one dimensional (1 D)plasmonic spectroscope with gradually changing periods from 400 to 200nm (from left to right). Scale bar, 21 μm. FIG. 14B is an opticalmicroscopy image of the plasmonic spectroscope illuminated with whitelight. FIG. 14C is an SEM image of the fabricated two dimensional (2D)spoke structure. FIG. 14D are optical microscopy images of the spokestructure illuminated with unpolarized light (center) and polarizedlight (four boxes).

FIG. 14A shows the fabricated device comprising gradually changingperiods from 200 to 400 nm that covers all colors in the visible range.When illuminated with white light, the structure becomes a rainbowstripe, with light emitting from the stack array, as shown in FIG. 14B.Plasmonic spectroscopes can disperse the whole visible spectrum in justa few micrometers distance, which is orders of magnitude smaller thanthe dispersion of the conventional prism-based device. This featureindicates that the color pixels formed by these structures provideextremely high spatial resolution for application in multiband spectralimaging systems. The inventive thin film stack structures can bedirectly integrated on top of focal plane arrays to implementhigh-resolution spectral imaging, or to create chip-based ultracompactspectrometers.

All structures discussed above have only one-dimensional linear slits(that is, a slit in the same direction). Here, a two dimensional 2Dmicroscale spoke structure (FIG. 14C) having slits in differentorientations and a gradual change of slit spacing is designed to furtherinvestigate spectral and polarimetric imaging responses. The spokestructure, as depicted in FIG. 14C includes 96 slits that form acircular ring. Each slit is 3 μm in length and 50 nm in width, and theinner and outer radius of the ring is about 3 and 6 μm. The spacingbetween neighboring slits changes from 200 nm from the centre to 400 nmtowards the outer edge of the ring, covering all colors in the visiblerange as the above linear spectroscope. FIG. 14D shows the polarimetricresponse of the spoke with different illuminations. When the spoke isilluminated with unpolarized white light, the transmitted light forms acomplete rainbow ring. However, if the illumination is the polarizedlight and the polarization is rotated, a clear dark region appears alongthe polarization direction. This is because of the absence of excitedSPs in the polarization direction, and thus the transmission isextremely low, which is consistent with the TM coupling mechanismdiscussed earlier. Therefore, this two-dimensional spoke structure, whenused with an imaging device, provides the ability for real-timepolarimetric information in spectral imaging, or alternatively can beused as a microscale polarization analyzer.

Human eyes typically have a resolution limit of about 80 μm at 35 mm.Therefore, plasmonic nanoresonators built in accordance with the presentteachings can build colored “super-pixels” that are only severalmicrometers in a lateral dimension and are much smaller than theresolution limit detectable by human eyes. At present, this lateraldimension is also 1-2 orders of magnitude smaller than the besthigh-definition color filters currently available. Furthermore, theseplasmonic devices have a longitudinal thickness that is 1-2 orders ofmagnitude thinner than that of colorant ones, which is very attractivefor the design of ultrathin panel display devices.

Besides the small dimensions, the nature of the polarization dependenceof plasmonic resonators is also attractive. This feature not onlybenefits the applications in LCD by eliminating the need of a separatepolarizer layer, but can also be used for extracting polarimetricinformation in spectral imaging.

Therefore, in accordance with various aspects of the present teachings,plasmonic nanoresonators are provided that disperse light with highefficiency spectrally. By arranging different resonators, arbitrarycolored patterns on a micrometer scale can be achieved. These artificialstructures provide an opportunity for display and imaging devices with ahigher spatial resolution, as well as much smaller device dimensionsthan those currently available. The design principle can be easilyexpanded to other wavelength ranges for multispectral imaging.

The MIM grating structure discussed above in accordance with certainaspects of the present teachings is fabricated using focused ion beam.But for large-scale production, nanoimprint lithography is contemplatedfor fabricating these structures over large areas. For this purposealternative structure designs that are better suited for suchfabrication processes, as recognized by those of skill in the art, arelikewise contemplated.

In FIGS. 15A-15C, a hybrid plasmon/waveguide structure with a singlepatterned Al layer on top of dielectric deposited on a glass substrateis provided. This robust, easy to fabricate structure can producevarious spectra depending on the desired application by altering theperiod, linewidth, and thickness of the Al grating or by changing thecontinuous dielectric layer. FIGS. 15A-15C are TM transmissionsimulation results for thick Al, high index dielectric, polarizingstructure (FIG. 15A) and a thin Al, lower index dielectric, hightransmission structure (FIG. 15B). FIG. 15C is a schematic of thestructure.

In FIGS. 15A-15B, two simulation sets are presented for differentapplications. The first structure which features a thick Al grating andhigh index dielectric (FIG. 15A) is targeted for polarizing applicationssuch as liquid crystal displays where transverse magnetic (TM) polarizedlight will be transmitted with 50% efficiency at the peak whiletransverse electric (TE) light is almost completely suppressed (<<1%).The second which has a thin Al grating and lower index dielectric (FIG.15B) shows a narrower spectral peak with high transmittance near ˜90% atthe peak wavelength.

FIG. 16A is a schematic of an alternative filter structure that containsa low index spacer layer (SiO₂) between metal grating (Al) and highindex guiding layer or dielectric layer (Si₃N₄). Adding a low refractiveindex layer between the metal grating and the waveguide layer providesadditional control parameter to obtain the desired spectra. FIG. 16Bshows experimental TM transmission results obtained for such a structurewith two different metal grating periods. FIG. 16B shows some initialexperimental results of blue and red color filters using such astructure with two different periods, 280 nm and 420 nm, respectively.The structures are fabricated by the nanoimprint-based technique.Reflective color filters can be made in a similar fashion.

In various aspects, methods of making plasmonic optical spectrumfiltering devices. Such methods include forming a resonator structurecomprising an electrically conductive metal nanograting subwavelengthstructure and an active material selected from a dielectric material ora photoactive material via a process selected from UV photolithography,nanoimprint lithography, focused ion beam processing, stamping or metaltransfer printing. In this manner, the electrically conductive metalnanograting subwavelength structure is formed that comprises at leasttwo openings capable of transmitting a portion of an electromagneticspectrum there through to generate a filtered output having apredetermined range of wavelengths via optical resonance.

Generally, contact printing involves transferring a material depositedon a prepatterned mold directly to a substrate with the application ofuniform pressure and temperature. This process has traditionally beenused to transfer metal layers to act as electrodes or masks forsubsequent etch steps. Devices with multiple layers, such asmetal-insulator-metal (MIM) structures of the present disclosure, can betransferred over using similar processing techniques. Using a SiO₂grating mold, a MIM pattern is transferred to a flexible polycarbonatesubstrate in order to create a thin film, reflective color filter. Thismethod can be used with roll-to-roll nanoimprint lithography and can beused to efficiently fabricate large-area structures on varioussubstrates for display applications.

In recent years, contact patterning methods such as nanoimprintlithography (NIL) have been increasingly used as low-cost processes tocreate nanosized features. NIL itself has been used for years toefficiently create nanoscale features over large areas, but scientistscontinued to research methods for large-scale, direct transfer ofnanostructures onto various substrates without the need for preciseetching processes. Microcontact printing or “soft lithography,”utilizing elastomeric stamps such as polydimethylsiloxane (PDMS) hasbeen used to transfer self-assembled monolayer patterns which caneffectively act as a mask for wet etching into various substratematerials ranging from metals to biomolecules. Metal transfer processingis a more recent method which involves metal films deposited on apatterned mold being transferred directly to a substrate due to chemicalbonding or higher surface adhesion to the substrate as opposed to themold. This technique has been used primarily to create etch masks forvarious applications, but transparent metal electrodes for organic lightemitting diodes and polymer solar cells have also been studied.

Metal transfer printing does not have to be limited to single layers.Multilayer devices such as metal-insulator-metal (MIM) structures can betransferred over to substrates using the same low pressure andtemperature processing utilized in traditional single layer procedures.Because this process can be applied with various types of materials,these structures can be used for applications such as metamaterials,plasmonic devices, and, thin film color filter structures. This processhas advantages over other methods used to fabricate the MIM structuresmentioned above because it can efficiently create devices on flexiblesubstrates as well as hard materials that have an intermediate polymerlayer and, by utilizing roll-to-roll NIL, can be used to create largearea devices.

Suitable methods for forming multiple layer optical device structures,including forming thin film active material layers (like dielectricmaterials or photoactive materials) or grated metal structures aredescribed in U.S. Pat. No. 7,648,767 to Fu, et al. entitled “MATERIALCOMPOSITION FOR NANO- AND MICRO-LITHOGRAPHY” and U.S. Patent PublicationNo. 2009/0256287 (application Ser. No. 12/421,333 filed on Apr. 9, 2009)to Fu, et al. entitled “UV CURABLE SILSESQUIOXANE RESINS FOR NANOIMPRINTLITHOGRAPHY,” International Patent Application No. PCT/US2011/27748 toPark et al. filed on Mar. 9, 2011 entitled “Methods of Making OrganicPhotovoltaic Cells Having improved Heterojunction Morphology,” and inU.S. Patent Publication No. 2009/0046362 (application Ser. No.12/100,363 filed on Apr. 9, 2008) to Guo et al. entitled “Roll To RollNanoimprint Lithography,” the relevant portions of each of which isincorporated herein by reference in its respective entirety.

MIM structures have been experimentally demonstrated to act as plasmonicresonators for filtering specific frequencies of light. Using focusedion beam fabrication, a plasmonic transmissive color filter has beenfabricated that can allow specific frequency bands to transmit while allothers are effectively blocked, effectively creating red, green, or bluecolors with sharper resonances and higher transmission intensities thanprevious reports of thin film color filtering. Such filters exploit aMIM structure, in which the top metal grating couples light with atransverse magnetic (TM) polarization (magnetic field parallel to thegrating lines) at certain frequencies into the plasmonic waveguidestructure while a bottom metal grating efficiently couples the light outto the far field.

Reflective color filter structures are provided based on similarprinciples for display applications, such as electronic readers andportable displays, which use natural light to create color images. Withthis objective in mind, an alternative embodiment related to MIM gratingstructure is shown in FIG. 17A. This structure couples in specificfrequencies of light but, with the absence of a bottom grating, thelight is effectively trapped and absorbed by the film while reflectinglight at all other frequencies. Using this principle, a cyan, magenta,or yellow (CMY) color spectrum is readily created by simply varying thegrating period. Simulated reflection spectra based on rigorous coupledwave analysis for TM polarized light are shown in FIG. 17B, whiletransverse electric (TE) polarized light is expected to be almostcompletely reflected at all wavelengths.

Example 4

These particular experiments are performed using a hard SiO₂ mold due tothe ease of creating deep structures, but this technique could certainlybe used with soft molds such as PDMS. The full process for transfer to aflexible substrate is outlined in FIGS. 18A-18D. To begin with, a SiO₂grating mold, approximately 1×1 in², is fabricated using conventionalNIL and pretreated with a fluoro-surfactant. The entire MIM stack isthen deposited onto the mold through subsequent evaporation andsputtering steps. First, a 5 nm layer of Au is deposited prior to a 30nm Al layer by e-beam evaporation, since Au has weaker adhesion to theSiO₂ mold, allowing for easier transfer. Following these steps, a 40 nmTiO₂ layer is also deposited using e-beam evaporation. Finally, a 100 nmAl layer is sputtered over the top of the mold. Sputtering is chosen toallow for a more continuous deposition of the Al film. Some infiltrationbetween the evaporated structures is still possible; however, asevaporation progressed, the line width gradually grew to allow a morecontinuous Al layer (bottom layer in FIGS. 19A-19B). FIG. 18B shows adiagram after the deposition processes. The mold is then placed incontact with a flexible, polycarbonate (PC) substrate and a uniformpressure and temperature of 50 psi and 160° C., respectively, areapplied for 5 min for an efficient transfer. The Nanonex NX2000imprinter is used for this step. After cooling, the PC substrate in FIG.18C is then peeled off the mold to realize the final structure, shown inFIGS. 19A-19B. It is important to note from the image that someparticulates deposited on the sidewalls during evaporation andsputtering can still be transferred over during this process but, asdiscussed later, this did not greatly affect any test results.

Reflection spectrum measurements are taken using a Filmetrics F20system. Angular reflection measurements are taken using a Woollamvariable angle spectroscopic ellipsometer (WVASE32). Reflectionsimulations are run using COMSOL MULTIPHYSICS. Scanning electronmicroscope (SEM) images are taken using a Philips XL30 EEG SEM.

A plasmonic color filter to reflect yellow light is fabricated. Toobtain optimized yellow filter, a 220 nm period mold with a large dutycycle (>80%) is used to create a thin film with high confinement of thecomplementary blue wavelengths while allowing all others to reflect. SEMimages of the top and cross-section view of the structure after transferfrom this mold onto PC are shown in FIGS. 19A-19B. The simulatedreflection spectrum is compared with that taken from the sample using anormally incident source/collector setup (Filmetrics F20). Newsimulations are performed to include material loss and index ofrefraction changes over the visible spectrum. These data are collectedby spectroscopic ellipsometry for thin films of both Al and TiO₂ todevelop a more accurate simulation model. The results are shown in FIGS.20A and 20B for TE and TM polarized light, respectively.

In FIGS. 20A-20D, there are three key features. First, the TM polarizedgraph shows very good matching between the experimental and simulateddata. The resonance wavelength is nearly the same as predicted and canbe easily changed by altering the period of the structure. The bandwidthof the results is larger than the simulations due to possible effectsfrom particulates and linewidth variations, but it is believed that thiscan be controlled with better deposition conditions and improved SiO₂mold fabrication. The second is the strong degree of coupling that isshown, because the resonance has a near 0% reflectance while values atother wavelengths climb to nearly 70%, providing a high color contrastbetween wavelengths. The third feature is the structure's apparentstrong coupling of TE polarized light at similar wavelengths to the TM.While not fully understood, it is believed that based upon some ofsimulation results, variance in the thickness of the dielectric layer aswell as the top Al layer can have strong effects on the TE spectrum.

Once again, the measured spectrum shows a wider bandwidth and furtherstudies will be performed to determine its exact nature. On the otherhand, the polarization insensitivity could be a highly desirable featurefor practical color filter applications. This lack of completepolarization dependence also provides a strong visual representation ofthis structure's capabilities. FIG. 20C shows a photograph of the frontside of the sample in unpolarized light with a distinct yellow color, ascompared with the grayish color from the sputtered Al as viewed from theback side of the sample [inset in FIG. 20C].

Another useful feature of the structure, especially for practicalapplications, is the low dependence of the samples' reflection spectraon the viewing angle between the source and collector. FIGS. 21A-21Cshows the reflection spectra for TE, TM, and unpolarized light taken atangles between the source and detector of 20°, 30°, and 50° using aspectroscopic ellipsometer. Data are collected using an Al mirror as areference with corrections added due to angle variance usingellipsometer data and the WVASE software. While there are some intensityvariations in the TE/TM spectra, resonance wavelength is not greatlyshifted and the unpolarized data show no major change in the resonanceor intensity throughout the measurements. This can also be seen in thephotograph in FIG. 20D that is taken at a relatively steep angle, butstill shows the yellow reflected color of the sample. This structureappears to be quite suitable for being utilized for reflectivetechnologies at various angles and demonstrates minimal angledependence.

Metal transfer lithography can provide a large-scale, inexpensive methodfor creating metal patterns on various substrates. By demonstrating asuccessful transfer of a uniform MIM structure over a 1×1 in², suchmetal transfer techniques can be extended to produce functional devicesand multilayer structures using a variety of combinations of materials.A yellow reflective color filter is fabricated with a distinctabsorption bandwidth and low angular dependence by simply depositingmultiple layers of material on a patterned mold and transferring onto aflexible PC substrate. Such processes are applicable to other periodgratings, for example, gratings that create cyan and magenta colors, fora working display. Other fabrication techniques that are contemplatedinclude top-down processes such as traditional NIL or newly developedmethods such as dynamic nano-inscribing. These structures can be readilyfabricated over large areas for a wide range of reflective displayapplications and that the successful transfer of MIM structures canprovide faster, cheaper fabrication of various other multilayerstructures.

In certain embodiments, a display device is provided that comprises adisplay pixel of a display screen comprising a plasmonic resonatorstructure for color filtering via optical resonance comprising anelectrically conductive metal grating structure and an active materialselected from a dielectric material or a photoactive material. Theelectrically conductive metal grating structure comprises at least twoopenings capable of transmitting a portion of an electromagneticspectrum generated by the display device to generate a filtered andpolarized output having a predetermined range of wavelengths.

In certain embodiments, like that shown in FIGS. 22-24, a display device500 is a liquid crystal display device. While a liquid crystal displayis used as an exemplary device for purposes of illustration, it shouldbe noted that the inventive filtering devices can also be designed forincorporation into a variety of display devices as recognized by thoseof skill in the art, including projection displays (such as usingdigital mirror technology, or liquid crystal on silicon (LCoS), eye-weardisplays, complementary metal-oxide-semiconductor (CMOS) image sensors,IR imagers, light emitting diodes, by way of non-limiting example.

In the context of FIG. 23, to the extent that elements of the LCDdisplay are commonly shared with a conventional design like that in FIG.1, the same reference numbers are used. A display pixel 501 includes afront polarizer 32, a first transparent conductor electrode 50, and aplurality of liquid crystals 64 capable of passing a predetermined rangeof polarized electromagnetic waves of wavelengths in response to anapplied voltage. A plasmonic resonator structure 550 is disposed on anopposite side of the plurality of liquid crystals 64 as the firsttransparent electrode 514, where the electrically conductive metalgrating structure 524 serves as a second transparent conductiveelectrode and the plasmonic resonator structure 550 (which may becombined with another polarizer sheet) serves as a rear polarizer.

Along the lower bottom side which receives light to be processed is afirst transparent substrate 24 (e.g., a glass substrate), while alongthe upper top side a front polarizer 32 is adjacent to a secondtransparent substrate 34. On the bottom side, a transparent lowerconductor 40 (or electrode) is disposed adjacent to a lower alignmentlayer 42 (having a surface morphology that induces a predeterminedorientation of liquid crystals upon application of current thereto).While not shown, similar to FIG. 1 external electrical contacts canconnect one or more metallic elements of the plasmonic resonatorstructure 550 to the first transparent electrode 50.

A liquid crystal compartment (60) is formed by one or more seals (notshown) in contact with the resonator structure 550 and an upperalignment layer 52 (that serves to provide an orientation to the liquidcrystals when electric potential is applied). The liquid crystalcompartment contains a plurality of liquid crystals 64. Two spacers 66are also disposed within the liquid crystal compartment 60. The uppertransparent conductor (or electrode) 50 is adjacent to the upperalignment layer 52. In certain variations, the resonator structure 550has a surface 510 with a morphology that induces a predeterminedorientation of the liquid crystals, much like the upper alignment layers(or the resonator structure 550 further includes an optional alignmentlayer as surface 510, not shown). The upper alignment layer 42 and loweralignment layer/resonator structure surface 510 facing the liquidcrystal compartment 60 have complementary surface morphologies thatinduce a preferred orientation for the liquid crystals 64 when voltageor electrical potential is applied to permit light to transmit androtate through the liquid crystals.

Thus, when electrical potential is applied to the upper and lowerconductors 50, 550, the liquid crystals 64 are oriented such that whitelight (generated within the display device) is permitted to pass throughthe resonator structure 550, into the liquid crystals 64, which is thentransmitted out of the front polarizer 32 to provide a filtered coloredlight. In the absence of electrical potential applied to theconductors/electrodes 50, 550, the liquid crystals 64 are randomlyoriented and no incident light passes through the liquid crystalcompartment. As shown in FIG. 22, LCD pixels (e.g., 501) of paneldisplay 500 frequently have an array of multiple color filterassemblies, for example including three adjacent red, green, bluefilters that can be tuned to provide one of a red-green-blue “RGB” colorupon selective activation. Notably, in the design of the LCD pixel 501,a conventional color filter and black matrix, as well as a rearpolarizer can be eliminated altogether.

A photoactive material optionally comprises an organic electron donormaterial comprising poly(3-hexylthiophene) (P3HT) and an organicelectron acceptor material comprising [6,6]-phenyl C₆₁ butyric acidmethyl ester (PCBM). The plasmonic resonator structure 550 furthercomprises a second electrically conductive metal electrode element 532and serves an organic photovoltaic device capable of generating aphotocurrent as well as the filtered and polarized output. In certainvariations, the organic photovoltaic device is capable of convertinglight energy received through at least a portion of the display screen.The display screen is integrated with the organic photovoltaic device.Furthermore, in certain embodiments, such a display screen is capable offunctioning as a touch screen.

In one exemplary embodiment, a detailed side profile is shown in FIG. 24(showing a portion of a pixel with a plasmonic resonator structure 550)that is a multi-layered dual-function photovoltaic-color filter devicecapable of producing desirable reflection colors and simultaneouslyconverting absorbed light 560 to generate electricity. Morespecifically, a reflective spectra of pixels are made with device (seeFIG. 22). Because the reflectance type color filters act similarly tothe color paint, e.g., absorbing light corresponding to specificwavelengths, but reflecting the others, in the embodiment, the CMY colorscheme is employed where cyan, magenta and yellow are three primarycolors. A transparent substrate 502 may comprise glass, such as silicondioxide (e.g., silica). A first conductive transparent electrode 504comprising indium tin oxide (ITO) is disposed in a stacking arrangementnear substrate 502 and as shown in FIG. 24 is disposed within a portionof an optional buffer layer 506 to enhance charge transport from thefirst electrode 504. The first electrode 504 may be an anode and maycomprise ITO or other transparent conductive materials. The buffer layer506 may comprise cesium carbonate (Cs₂CO₃). The multi-layered structure550 may comprise one or more photoactive organic semiconductor materials508. The semiconductor layer 508 is optionally adjacent to a secondbuffer layer 510, such as vanadium oxide (V₂O₅). The photoactivesemiconductor material layer 508 comprises a material like an organicelectron donor, such as poly(3-hexylthiophene) (P3HT), combined with anorganic electron acceptor, such as [6,6]-phenyl C₆₁ butyric acid methylester (PCBM) to form a combined layer of mixed P3HT:PCBM. While notlimiting the present disclosure to any particular organic materialsystem, for purposes of illustration, a conjugate polymer system ofpoly(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester(PCBM) shows a bulk heterojunction (BHJ) P3HT:PCBM blend used as anexemplary model system.

A second electrode 512 is disposed over the second buffer layer 510along the surface of the photoactive material layer 508. The secondelectrode 512 may be a transparent cathode and may comprise aluminum,gold, silver, copper, or other conductive materials. The secondelectrode 512 comprising aluminum (a cathode) can be in the form of acontinuous film that sandwiches the active organic semiconductors layers(buffer layers 506, 510) and photoactive material layer 508). In certainvariations, either the first electrode 504 or the second electrode 512may be a nanograting, although in FIG. 24 only second electrode 512 is ananograting. The conductive metal structures can serve as an electrodeto tune an orientation of a plurality of liquid crystals in a liquidcrystal display. A filtered output 570 is generated from the plasmonicresonator structure 550 at a predetermined range of wavelengths, whichis capable of serving as both a color filter and a polarizer in the LCDdisplay pixel in FIGS. 22-23.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A plasmonic optical spectrum filtering devicecomprising: a resonator structure comprising an electrically conductivemetal grating structure, a buffer layer adjacent to the gratingstructure, and a dielectric material adjacent to the buffer layer,wherein the electrically conductive metal grating structure comprises atleast two openings capable of transmitting a portion of anelectromagnetic spectrum to generate a filtered output having apredetermined range of wavelengths via optical resonance.
 2. Theplasmonic optical spectrum filtering device of claim 1, wherein theresonator structure further comprises an electrically conductive metalelement disposed along an opposite side of the dielectric material. 3.The plasmonic optical spectrum filtering device of claim 1, wherein theelectrically conductive metal grating structure comprises a metalselected from the group consisting of: gold, aluminum, silver, copper,and combinations thereof.
 4. The plasmonic optical spectrum filteringdevice of claim 1, wherein the resonator structure further serves as apolarizer device.
 5. The plasmonic optical spectrum filtering device ofclaim 1, wherein a periodicity defined by the at least two openings ofthe electrically conductive metal grating structure determines thepredetermined range of wavelengths that is transmitted in atransmission-type filtering device or reflected in a reflection-typefiltering device.
 6. A display device comprising a display screenincorporating the plasmonic optical spectrum filtering device of claim 1as a pixel.
 7. The plasmonic optical spectrum filtering device of claim1, wherein the predetermined range of wavelengths of the filtered outputis in the visible light range and has a color selected from the groupconsisting of: cyan, yellow, magenta, red, green, blue, and combinationsthereof.
 8. The plasmonic optical spectrum filtering device of claim 1,wherein the dielectric material is selected from the group consistingof: silicon nitride (Si₃N₄), zinc selenide (ZnSe), zinc oxide (ZnO),zirconium oxide (ZrO₂), and titanium oxide (TiO₂).
 9. The plasmonicoptical spectrum filtering device of claim 1, wherein the buffer layercomprises a material adjacent to the grating structure selected from thegroup consisting of: a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) material, cesium carbonate (Cs₂CO₃),silicon dioxide (SiO₂), zinc oxide (ZnO), vanadium pentoxide (V₂O₅),nickel oxide (Ni₂O), Molybdenum oxide (MoO₃), and combinations thereof.10. A plasmonic optical spectrum filtering device comprising: aresonator structure comprising a first electrically conductive metalgrating structure, a dielectric material, and a second electricallyconductive metal grating structure, wherein the first electricallyconductive metal grating structure and the second electricallyconductive metal grating structure together define a subwavelengthgrating structure comprising at least two openings capable oftransmitting a portion of an electromagnetic spectrum to generate afiltered output having a predetermined range of wavelengths via opticalresonance, wherein the at least two openings in the subwavelengthgrating structure have at least one dimension that is less than thepredetermined range of wavelengths.
 11. The plasmonic optical spectrumfiltering device of claim 10, wherein the resonator structure furthercomprises an electrically conductive metal element disposed along anopposite side of the dielectric material.
 12. The plasmonic opticalspectrum filtering device of claim 10, wherein the resonator structurefurther serves as a polarizer device.
 13. The plasmonic optical spectrumfiltering device of claim 10, wherein a periodicity defined by the atleast two openings of the subwavelength grating structure determines thepredetermined range of wavelengths that is transmitted in atransmission-type filtering device or reflected in a reflection-typefiltering device.
 14. A display device comprising a display screenincorporating the plasmonic optical spectrum filtering device of claim10 as a pixel, wherein the predetermined range of wavelengths of thefiltered output is in the visible light range and has a color selectedfrom the group consisting of: cyan, yellow, magenta, red, green, blue,and combinations thereof.
 15. The plasmonic optical spectrum filteringdevice of claim 10, wherein the first electrically conductive metalgrating structure and the second electrically conductive metal gratingstructure independently comprise a metal selected from the groupconsisting of: gold, aluminum, silver, copper, and combinations thereof,and the dielectric material is selected from the group consisting of:silicon nitride (Si₃N₄), zinc selenide (ZnSe), zinc oxide (ZnO),zirconium oxide (ZrO₂), and titanium oxide (TiO₂).
 16. A plasmonicoptical spectrum filtering device comprising: a resonator structurecomprising an electrically conductive metal grating structure, a bufferlayer adjacent to the grating structure, and a dielectric material,wherein the electrically conductive metal grating structure comprises atleast two openings capable of transmitting a portion of anelectromagnetic spectrum to generate a filtered output having apredetermined range of wavelengths in a visible light range via opticalresonance, wherein a periodicity defined by the at least two openings ofthe electrically conductive metal grating structure determines thepredetermined range of wavelengths that is transmitted in atransmission-type filtering device or reflected in a reflection-typefiltering device, wherein the predetermined range of wavelengths has acolor selected from the group consisting of: red, green, blue, andcombinations thereof, for the transmission-type filtering device and thepredetermined range of wavelengths has a color selected from the groupconsisting of: cyan, yellow, magenta, and combinations thereof, for thereflection-type filtering device.
 17. The plasmonic optical spectrumfiltering device of claim 16, wherein the resonator structure furthercomprises an electrically conductive metal element disposed along anopposite side of the dielectric material.
 18. The plasmonic opticalspectrum filtering device of claim 17, wherein the electricallyconductive metal grating structure is a first electrically conductivemetal grating structure and the electrically conductive metal element isa second electrically conductive metal grating structure, wherein thefirst electrically conductive metal grating structure, the dielectricmaterial, and the second electrically conductive metal grating structuretogether define the at least two openings through the resonatorstructure that generates the filtered output having the predeterminedrange of wavelengths.
 19. The plasmonic optical spectrum filteringdevice of claim 16, wherein the at least two openings in theelectrically conductive metal grating structure have at least onedimension that is less than the predetermined range of wavelengths. 20.A display device comprising a display screen incorporating the plasmonicoptical spectrum filtering device of claim 16 as a pixel.